[feature request] allow building using system spirv-cross

[Apteryks]

Description

Currently, vulkan supports depend on using the 22 MiB bundled spirv-cross source under deps/.

Expected behavior

I'd like a switch to --enable-systemspirv_cross (or should it be --disable-builtinspirv_cross? I wanted to implement it but was confused about that).

Actual behavior

Only bundled source can be selected. If disabled, vulkan backend is no longer available.

• RetroArch: latest from master

Environment information

• OS: Guix System (GNU/Linux)
• Compiler: GCC 11